I realize this is straying off topic, but why would an entrepreneur hamstring him/herself by agreeing to such a clause? It appears to me that such a clause benefits the VC in a big way, but puts a significant amount of undue pressure on the entrepreneur. How am I supposed to know if I am getting the best deal if I am barred from taking the offer into the marketplace?

Yes, it is a pro-investor clause. You would agree to it in the situation where you feel you are satisfied with the term sheet (and who its from) and are ready to commit.